Production of ethylene



March 15, 1960 w. R. NISBET ET Al. 2,928,886

PRODUCTION OF ETHYLENE Filed Aug. 19, 1955 name: I

INVENTOR wnuml a. mass-rwnum u. was

I I m STEAM ATTORNEY PRODUCTION OF ETHYLENE William R. Nisbet, Texas City, and William H. Agee, La Marque, Tex., assignors to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware Application August'19, 1955, Serial No. 529,409

3 Claims. (Cl. 260-683) The present invention pertains to a process for conducting thermal conversions of gaseous material. The invention is particularly concerned with the quenching of cracked gas rich in ethylene resulting from the pyrolysis of hydrocarbons.

The large family of industrially important chemicals and plastics which can be manufactured from ethylene has led to increased demand for this highly reactive unsaturated hydrocarbon. An established method for making ethylene for use in chemical manufacturing involves high-temperature pyrolysis or cracking of a hydrocarbon feed stock such as gas oil or saturated light hydrocarbons such as ethane, propane, and butane. Several practical difiiculties arise, however, .in conducting such a process. The temperatures required for the reaction are very high and the optimum time during which the feed stocks may be held at the elevated conversion temperature is very short, in the order, for example, of from a few seconds down to a few hundredths of a second. Under the prevailing high temperature conditions, lengthened reaction periods allow secondary reactions, such as decomposition and polymerization or other reactions of the product, to predominate resulting in decreased yield and a troublesome accumulation of tarry substances and finely divided coke in the system. Accordingly, the cracked g'as product must, upon completion of the desired conversion, be shock-cooled or quenched immediately.

Many methods have been proposed for carrying out the quenching operation. The more common of these include quenching with liquids such as water or hydrocarbon oils since passage of the hot cracked gases over wateror air-cooled heat transfer surfaces such as metal plates is, of course, inefiicient in bringing temperatures down rapidly. Certain defects, however, are inherent in the use of such media as have heretofore been employed. While water is eminently suitable because of its latent heat characteristics and represents the most economical operation both from the standpoint of capital equipment required and material cost, serious problems are presented when the cracked gas is quenched directly in a spray of water. It is virtually impossible to avoid altogether the formation of some tarry substances and some finely divided coke in the pyrolytic reaction. With a water quench, therefore, tar emulsions are formed which present a serious disposal problem. In addition, much of the coke present being hydrophobic in nature is not washed down by water and as a consequence it accumulates in the equipment plugging up orifices, passages, heat exchangers, automatic relief valves, etc.

Certain hydrocarbon oils, on the other hand, are superior to water as a quench medium in that they exert a wetting action on the tar and coke slurrying these materials and keeping them in motion so that they do not settle out in undesirable and inaccessible spots in the system. However, oil is a more expensive quench medium than water and, because of its poorer latent heat characteristics, a significantly larger quantity of oil is required for the quenching operation as compared to waf ce ter, thus necessitating more and larger equipment as well as additional process or handling steps. 1

A major object of this invention, therefore, is the provision of an improved practical process for conducting gaseous conversions of the type requiring very high temperatures and rapid quenching of the reaction products.

Another object of the invention is to accomplish quenching of the reaction products from the pyrolysis of hydrocarbon stocks in an effective manner without encountering certain defects found in conventional methods.

A more specific object of the invention is the provision of an improved process for making ethylene by the py-.

rolysis of light saturated hydrocarbons in which the quench step is carried out without encountering the difliculties which occur with the usual water and oil quench media.

These and other objects and advantagesof the invention will be obvious from the following detailed description thereof. According to the invention, a hot cracked gas efiluent is quenched by introducing it into a suitable zone or tower wherein: it is sprayed with an oil-in-Water emulsion con-I per part of vaporized propane.

taining from about 5 to about 40% of oil. By using such an oil-water emulsion as a quench medium, the ad; vantages of oil quenching combined with those of water quenching are realized without any of the attendant difficulties experienced in those systems wherein oil or water 7 is used separately as discussed in some detail above. With a water quench alone, for example, shut-downs for the purpose of cleaning coke out of the equipment were necessitated on the average of about once every two or three months. Since employment of an oil-water quench was begun, no such shut-down s'have been required and inspection after twoto three-month periods of operation has indicated the system to be essentially free of coke build-up.

For further understanding of the invention, the follow: ing description of the process as applied in the production of ethylene from the. cracking of a propane charge stock is presented with reference to the accompanying diagrammatic flowsheet of Figure I wherein a preferred arrangement of suitable equipment for carrying out the invention is presented.

The propane charge stock from storage facilitiesim dicated at 10 is passed through a vaporizer 11 where it is heated to approximately C. and thence via line 12 to a cracking furnace 13 designed in accordance with principles well known in the art. While the undiluted hydrocarbon maybe pyrolyzed as such, it is preferable to employ an inert carrier gas to lower the partial pressure of the propane in the cracking zone and to improve heat transfer. Steam is admixed, therefore, with the vaporized propane at the point indicated before the gas enters the furnace and generally in an amount such that there is from about 0.05 to about 3.0 parts by weight of steam The cracked gas efiiuent containing approximately 20-25 mol percent ethylene together with hydrogen, methane, ethane, propane, propylene, and small amounts of higher hydrocarbons leaves the furnace at approximately 800 C. and 5 p.s.i.g. pressure via line '14. The efiluent is then passed into the lower section of the quench tower 15 wh-re it is immediately shock-cooled to a temperature of approximately C. by contact with an oil-water emulsion ejected from sprays in the top of the tower, pumped from a quench medium surge tank 16 to which water, as steam condensate, and oil are charged. The oil-water emulsion contains approximately 5-15% oil. Typical of the oils which may be used is one which is aromatic in character containing only 17.2% parafiins, has a specific gravity of 68 F. of 0.9212, a viscosity of 32.2 Saybolt sec., and upon distillation has an initial boiling point of 350 F., a 50% boiling point of 442 F. The oil-water emulsion quench 'med'ium together with condensable light oils formed in the cracking operation is continuously withdrawn from the bottom section of the quench tower at a point below the cracked gas entry line through line 17 and passed to the surgetank 16. This oil-water mixture is-then pumped from the surge tank by means of the .pump 18 which also provides the agitation necessary to keep the-oil-water mixture emulsified, wthrough-the shell side of the heater;- changer 19 employing cooling tower water on the tube side wherein it is cooled to a temperature in the range from 40-50" C. and recycled for injection into the quench tower 15 through the spray devices, 20, 21, and 22. Heavy tarry matter settles to the bottom of the quench tower 15 and is withdrawn through line 23 periodically for disposal. The cooled cracked gas rises and leaves the topof the quench tower through line 24, is further cooled and-- mayjbe passed directly to a suitable purification system.

In order to control the character and composition of the oil-water emulsion which may be altered to some extent by the accumulation of heavy oil, coke, or tar which is necessarily carried over into the quench medium surge tank, a slipstream is withdrawn continuously from the tank through line '25 and pumped to a g-ravity vseparator '26 for clean up. In the separator coke, heavy oils or tarry materials are settled to the bottom and withdrawn while the clean oil-in-water emulsion overflows through the line 27 by gravity back to the surge tank 16 where it is recycled to the quench tower.

For optimum eifectiveness, the oil-in-water emulsion should contain from about 5 to about 40%"by volume of oil and preferably from about 5 to about of oil. Suitable oils are those which are aromatic in character since they must be compatible with the-oils or liquid products formed by condensation from the cracked gas being treated. In addition, the oils to be used should be characterized by high latent heat values, a specificgravity of slightly less than 1, being in the range, for example, from 0.9to about 0.98 at 68 F. and having a boiling range from approximately 350 to 700 F.

While the example given has been directed specifically to the production of ethylene from the cracking of a propane charge stock, the invention is not to be considered as limited to this adaptation. The novel method of quenching herein described is applicable in any process wherein a hydrocarbon or hydrocarbon mixture is cracked at elevated temperatures to form olefins. Ethane, butane,

refinery gas and liquid fractions such kerosene, ,gas

oil and fuel oil may all be used.

What is claimed is:

1. In a continuous process for pyrolytic conversion of hydrocarbon stocks at elevated temperatures to gaseous hydrocarbon products which are unstable at the elevated conversion temperatures, the method of quenching the hot cracked gas which comprises passing said hot cracked gas through a quench zone in contact with a relatively cool oil-in-water. emulsion, vSaid .oil-inwa-ter emulsion containing from about 5% to about 40 by volume of an aromatic oilcharaeterized by a high latent heat of vaporization, a specific gravity in the range f-rom about 0.9 to about 0.98 at 68 F., and a boiling range from about 350 to about 700 F 2. in a continuous process of producing ethylene by pyrolysis of hydrocarbons at elevated temperatures, the method of quenching the hot cracked gas which comprises passing said hot cracked-gas through a-q-uench zone in contact with a relatively cool oil-in-water.emulsion.

3. In a continuous process of producing ethylene by pyrolysis of propane at elevated temperatures, the method of quenching the hot cracked gas which comprises passing said gas through a quench zone in'contact with a relatively cool oil-in-water emulsion, said oil-in-wateremuls'ion preferably containing from about 5 to about 15% -by volume of an aromatic oil characterized by a high latent heat of vaporization, a specific gravity in the range from about 0.9 to about 0.98 at 68 F, and a boiling range from about 350 to about 700 F., said oil-in-wa-ter emulsion containing from about 5% to about 40% by volume of an aromatic oil characterized" by a high latent heat of vapor ization, a specific gravity in therange from about 0.9ato

about 0.98 at 68 F, and a boiling range from about 350 to about 700 F.

ReferencesCited in the file of this patent UNITED STATES PATENTS De Rosset Feb. 12, 1957 White May 1, 1934 UNITED STATES PATENT OFFICE CERTIFICATE 0F CQRRECTION Patent No, 2,928,886 March 15, 1960 William R. Nisbet et a1.

It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2 line 61, for "whre" read where line 69 Q for "of" first occurrence, read at Signed and sealed this 30th day of August 1960,

(SEAL) Attest:

ERNEST W, SWIDER ROBERT C. WATSON Attesting ()ificer Commissioner of Patents 

1. IN A CONTINUOUS PROCESS FOR PYROLYTIC CONVERSION OF HYDROCARBON STOCKS AT ELEVATED TEMPERATURES TO GASEOUS HYDROCARBON PRODUCTS WHICH ARE UNSTABLE AT THE ELEVATED CONVERSION TEMPERATURES, THE METHOD OF QUENCHING THE HOT CRACKED GAS WHICH COMPRISES PASSING SAID HOT CRACKED GAS THROUGH A QUENCH ZONE IN CONTACT WITH A RELATIVELY COOL OIL-IN-WATER EMULSION, SAID OIL-IN-WATER EMUL- 